The Origins of Hot Subdwarf Stars as Illuminated by Composite- Spectrum Binaries Michele A. Stark (Penn State University) Advisor: Richard A. Wade Thursday Jan 13, 2005AAS 205 th Meeting, San DiegoSession 170 – White Dwarfs and Hot Subdwarfs

Plan of Attack I.What a hot subdwarf is II.Why hot subdwarfs are cool III.How hot subdwarfs gets hot IV.How we are cool: a. 2MASS b. Spectroscopy c. Nature of the companions V.What does it all mean?

I. Hot Subdwarf Classification Subdwarf B (sdB) Subdwarf O (sdO) T eff (K) ~20,000–30,000 > 30,000 Main Spectral Feature Hydrogen Helium L & log g Dispersion Small Large Evolutionary Paths to T, L, log g Single Multiple

II. Why hot subdwarfs are cool… Galactic field Extended Horizontal Branch (EHB) stars  M tot = ~0.5 M   M env < ~0.01 M  Source of “UV-upturn” (or “UVX”) seen in old stellar populations EHB stars have been imaged in UVX galaxies Brown et al. 2000, ApJ, 532, 308 M32 STIS, UV 25”

III. How a hot subdwarf gets hot: Binary Formation Scenario Current scenario for Galactic field sdB formation Idea: Binary interactions  significant mass loss (i.e., Mengel et al. 1976; Han et al. 2002, 2003) Consequence: nearly all sdB should be found in “close” binary systems Known Binary Companions:  white dwarfs (P ~ hours  30 days)  handful of very low mass dM (M ~ 0.08 M  )  GK-type stars (“long period,” >years = unknown)

IV. How we are cool Idea:Idea:  Use late-type companions to better understand sdBs (L, d, population, etc.) How?:How?:  2MASS  sdBs w/ unresolved late-type companions (J [1.2  m], H [1.65  m], and K s [2.2  m])  Spectroscopically study the companions Goals & Future Significance:  Identify nature & physical parameters of the companions  Determine physical parameters of sdBs in composites  Provide limits for binary formation channels

IVa. 2MASS Composites Two distinct “clumps” in 2MASS (J-K S ) vs. (J-H) color-color space:  “Single” colored  Composite colored Composite “Single”  Blue Red 

IVa. 2MASS Composites Companions:  Late F, G, K only  No dM or early F (most pronounced in the best photometry sample,  (Q) < 0.1) Magnitude Limited:  60±4% “single” (357)  40±4% composite (237) “Volume” Limited:  75±4% “single” (309)  25±4% composite (104) “Single” Composite  BlueRed  All  (Q)  < 0.2  (Q)  < 0.1

IVb. Spectroscopic Sub-Samples Hot Subdwarfs  21 single  53 composite V < 13.5 J-K S > +0.5 Single Composite

IVb. Spectroscopic Sub-Samples “Standards”  84 stars  Pop I, MS and SG  M V from HIP 

IVb. Spectra Obtained KPNO 2.1m GoldCam, McDonald 2.7m LCS ~4600–8900 Å, ~1.3 Å/pix Single Standard Composite

IVc. Refining Model Grid Fit standard star parameters Use fits to interpolate grid Compare grid to observed hot subdwarf composites Standards Model

IVc. Refining Model Grid Take interpolated EWs/colors and predict the EWs/colors if diluted by a sdB Unable to flux calibrate  used Kurucz flux distributions to compute dilutions at different wavelengths Compare predicted diluted EWs/colors to observed composites…

IVc. Individual Fits Best fit comp:  (B-V) c  0.95  M V,c  6.25  ~K2 Best fit sdB:  (B-V) sdB   M V,sdB  4.10

IVc. Individual Fits Same sdB, different comp.  (B-V) sdB ~  M V,sdB ~ 4.3  (B-V) c ~ +0.7, +0.9  M V,C ~ 5.3, 6.0 (~F7 V) (~K1 V)

IVc. Individual Fits Same companion, different sdB  (B-V) c ~ +0.9  M V,C ~ 6.0  (B-V) sdB ~-0.22,-0.26  M V,sdB ~ 4.1, 4.7 (cool/bright sdB) (hot/faint sdB)

IV. Preliminary Fit Summary Majority of companions are best fit with main sequence A few appear to be subgiants Cover the range of subdwarf temperatures (no temperature bias or trends evident)

IV. Preliminary Fit Summary Work remains to be done over next ~6 months:  Refine model grids and model-matching technique  Investigate possible reddening effects  Verify results do not change for terminal-age EHB

V. What does it all mean? Goals & Future Importance:  Identify nature & physical parameters of the companions  Determine physical parameters of sdBs in composites  Provide limits for binary formation processes  Refine binary formation rates for Algols, SN Ia, CV, etc.  Refine population synthesis models for UV-upturn

V. What does it all mean? 2MASS Results:  Fraction of composite sdB in current samples: 40% (magnitude limited) 25% (volume limited)  There are no companions of dM type or earlier than ~mid-F type in current sdB samples (Preliminary) Spectroscopy Results:  Most late-type companions are fit by main sequence stars  There are a handful of subgiants

Special Thanks to: G. B. Berriman T. Bogdanovic J. Ding R. Ganguly K. A. Herrmann K. T. Lewis A. Narayanan NASA-GSRP NOAO Graduate Travel Support Pennsylvania Space Grant Consortium Sigma Xi Grants-in-Aid of Research Zaccheus Daniel Foundation for Astronomical Science